An electrically driven panel, such as an electroluminescent (EL) panel, is used to display indicia in response to appropriate electrical signals imposed upon its electrodes.
The typical electrical model of an electroluminescent (EL) display panel includes horizontal (row) and vertical (column) electrodes of finite resistance with an interelectrode capacitance evenly distributed along the length of each electrode, with such a model being illustrated in FIG. 1. A row driver working through row switches appropriately drives the row electrodes. A column driver (not illustrated) drives the column electrodes through a bank of column switches. Separate power supplies (not illustrated) typically are provided to separately power the row and column drivers.
The row driver for such an EL panel has two separate modes of operation. In the write mode, each row typically is pulsed sequentially down to -160 V (volts), while the column voltages determine the pixel intensities along the row being addressed. In the refresh mode, all rows are typically simultaneously pulsed up to +220 V, while the columns are all maintained at zero volts. A typical, prior art row drive wave form is graphically illustrated in FIG. 2, showing the relative voltage levels which occur in the row driver through each cycle.
The column electrodes have a relatively high resistance, since they are generally made of a thin-film transparent material. For this reason each column driver sees a load equivalent to a delay line. Each row electrode, however, has a low resistance, and for all practical purposes the row driver sees a purely capacitive load. This type of load enables the use of the resonant row drive scheme of the present invention.
It is noted that some EL panels do not exhibit purely capacitive loads or characteristics, but even these types of panels effectively could be made into a purely capacitive load by adding external capacitance to the panel. Such a panel, so modified or supplemented, would then present an essentially capacitive load, then allowing the use of the resonant row drive scheme of the present invention.
FIG. 3 schematically shows a typical row driver circuit of the prior art. Initially all switches are "off" except switch S.sub.1. In order to produce a -160 V write pulse on the EL panel's row electrodes, schematically illustrated in the circuit as the capacitive load C.sub.panel, switch S.sub.4 is turned "on," coupling the panel C.sub.panel to -160 V through switch S.sub.1, diode D.sub.3, and switch S.sub.4.
If switch S.sub.4 is released and switches S.sub.5 and S.sub.6 are activated, the capacitative row panel load C.sub.panel switches back to zero volts. Now, with switch S.sub.1 "off" and S.sub.2 activated, the bottom or negative side of the capacitor C.sub.1 is raised to +60 V, while the top (positive side) of capacitor C.sub.1 is raised to +220 V, since capacitor C.sub.1 already had a charge of +160 V across it. A +220 V refresh pulse can now be generated at the panel load C.sub.panel by switching switch S.sub.3 "on." When the switch S.sub.3 is released and switches S.sub.5 and S.sub.6 are activated, the panel load C.sub.panel again will return to zero volts.
The foregoing describes the typical prior art row driver system for EL panels, and such a circuit has significant disadvantages. It requires dual regulated, relatively high voltage supplies (-160 V & +60 V) to drive the EL panel's row electrode load C.sub.panel. Also, such a prior art drive scheme is not energy efficient.
Additionally, in the prior art design an electrode short on the panel could destroy expensive driver integrated circuits (ICs) and cause the electrode to burn out on the panel itself.